A fracture splitting method has been used as a producing method of connecting rods for automobile engines.
A connecting rod is split into two pieces at its large end portion connected to a crackshaft, and in general, one of these pieces including a rod portion connected to a piston is referred to as a “rod body”, and the other including a semi-circular portion at the large end portion is referred to as a “cap” or the like.
The fracture splitting method is also called as a “cracking method” that hot-forges a material into a shape with the rod body and the cap integrated, that is, the same shape as that when assembled to the crankshaft, and thereafter, the hot-forged material is split into two portions (parts) of the rod body and the cap. Splitting into two parts is carried out as if they are fractured by applying impact load; thus this is referred to as the “fracture splitting”.
The fracture splitting method eliminates hot-forging the rod body and the cap separately, and allows tight fitting between fine recesses and projections existing on brittle fracture surfaces which are formed at the time of splitting; therefore, it is possible to eliminate “positioning pins” embedded for preventing positioning deviation between the rod body and the cap. Specifically, the fracture splitting method can significantly decrease producing processes, thereby greatly reducing production cost.
In order to apply the fracture splitting method, it is essential to have a property of being split in a brittle manner by applying impact load. In general, a cutout is so introduced as to generate stress concentration at a portion which is desired to be fractured, and thus deformation of a product is concentrated in the vicinity of the fracture portion; and if a degree of plastic deformation is great until the fracture occurs, both pieces of the product cannot be smoothly fitted to each other at their fracture surfaces after being split. If a number of voids unique to ductile fracture occur on the fracture surfaces, this hinders smooth fitting, and thus it is preferable to attain brittle fractures like “cleavage fracture”, which helps to attain smooth fracture surfaces. Specifically, it is preferable to split the product with almost no energy of impact stress absorbed.
High fatigue strength is also required in connecting rods, and thus non-post-heat treated steel that has high strength as it is hot forged is desired.
Various studies have been conducted on non-post-heat treated steel excellent in fracture splitting performance with low toughness and high strength for years. Particularly, investigation has been conducted on high-strength low-toughness non-post-heat treated steel having a duplex microstructure of ferrite and perlite (referred to as “ferrite+perlite microstructure”, hereinafter) that improves a disadvantage of non-post-heat treated steel having a perlite microstructure containing carbon of approximately 0.7% in mass %, as described in Patent Document 1, which has been practiced as the steel for a cracking connecting rod in Europe for the first time. This is because perlite is harder than ferrite, and inherently has a low toughness property, so that perlite is preferable for fracture-splitting, but endurance ratio (ratio between tensile strength and fatigue limit) is so small that it is hard to attain high fatigue strength, and perlite is also poor in drill workability for drilling bolt holes due to its high hardness.
V-based non-post-heat treated steel to which vanadium is added is representative of non-post-heat treated steel including the ferrite+perlite microstructure and having high strength. V-based non-post-heat treated steel is widely applied to products for mechanical structures in which high strength and high toughness are required, but low toughness suitable to cracking connecting rods cannot be attained if V-based non-post-heat treated steel is used as it is. To address this disadvantage, there have been suggested and disclosed various methods of providing V-based non-post-heat treated steel with a low toughness property.
In an invention described in Patent Document 2, phosphorus that segregates at grain boundaries to encourage brittleness is positively added, and the phosphorus content is defined so as to enhance the fracture splitting performance. In an invention described in Patent Document 3, the form and the number of sulfide-based inclusions are controlled so as to enhance the fracture splitting performance. Inventions described in Patent Documents 4 to 6 are directed to attaining non-post-heat treated steel having a high-strength low-toughness property by adding titanium. Patent Document 4 describes that TiN inclusions each having a diameter of 5 μm or more are dispersed, thereby enhancing a cracking property, and obtaining appropriate rough fracture surfaces. Patent Document 5 describes that the fracture splitting performance can be improved by controlling the form and the number of sulfide-based inclusions, and controlling “amount of effective Ti” defined based on the titanium content and the nitrogen content, that is, “amount of remaining Ti obtained by subtracting TiN from amount of Ti in the steel” to be 0.003% or more in mass %. In the invention described in Patent Document 6, non-post-heat treated steel having a low-toughness and a low-ductility along with a free-cutting property is attained in such a manner that Ti is added, Zr is compositely added along with Ti if necessary, and Ca of 0.0005 to 0.01% is further added, and at this time, balance of amount of Ti, Zr, and S in the steel is specified so as to generate sufficient amount of (Mn, Ca) S that is complex sulfide of Mn and Ca even after Ti combines, or Ti and Zr combine with S to generate sulfide.